add the roms to their respective folders (in the retropie/roms folder)

plug it back into the raspberry pi

wait for it to finish blinking

refresh emulationstation by choosing restart emulationstation from the start menu

Transfer Games with WinSCP method

Download WinSCP to transfer the files to the raspberry.
Your Raspberry and your computer must be connected on the same network.

Open WinSCP

Host name: retropie

User name: pi

password: raspberry

Press Login Button

On the right side, open the folder: home/pi/retropie/roms

Open the roms folder you want in the list, example: NES

Drop your NES roms in this folder

Refresh emulationstation by choosing restart emulationstation from the start menu.

After you’ve added your roms you need to restart emulationstation in order for them to show up.
You can restart emulationstation from the start menu, or by rebooting your pi with this command:
sudo reboot